This application relates to systems, devices, and techniques that implement data communications in single-wavelength-channel and multiple-wavelength-channel passive optical network systems.
A passive optical network (PON) is an optical network architecture based on point-to-multipoint (P2MP) topology in which a single optical fiber and multiple passive branching points are used to provide data communication services. A PON system can facilitate user access with a service provider communication facility to access telecommunication, information, entertainment, and other resources of the Internet. A PON system can include a central node, called an optical line terminal (OLT), which can be in connection with a single or multiple user nodes called optical network units (ONUs) via a passive optical distribution network (ODN). An ONU can be located at or near the access user's premises. An OLT can be located at the access provider's communication facility (central office). In a conventional PON system based on the time-division multiplexing/time-division multiple access (TDM/TDMA) principles, such as G.984 G-PON or G.987 XG-PON, the OLT operates on a single downstream wavelength and a single upstream wavelength. The plurality of the ONUs connected to the OLT over the ODN share the same downstream and same upstream wavelength.
A G.987 XG-PON system operating at the line rates of 9.95328 Gb/s downstream and 2.48832 Gb/s upstream has a designation of XG-PON1. A G.987 XG-PON system operating at the symmetric line rates of 9.95328 Gb/s downstream and upstream has a designation of XG-PON2. The nominal shorthand notation for 9.95328 Gb/s is 10 G, and the nominal shorthand notation for 2.48832 Gb/s is 2.5 G.
In a multi-wavelength passive optical network (MW-PON), multiple OLTs each operating on a unique downstream wavelength and unique upstream wavelength are connected to one and the same ODN via a wavelength multiplexor (WM), and over the said ODN are connected to a plurality of ONUs. A combination of one downstream wavelength and one upstream wavelength associated with a given OLT forms a bi-directional wavelength channel. Multiple downstream wavelengths reach each ONU; however, each ONU is capable of receiving and processing only one downstream wavelength and generating only one upstream wavelength at any given time. An ONU may be designed to operate on a specific pair of downstream and upstream wavelengths, in which case it is a fixed ONU, or it may be capable of changing its downstream and upstream wavelength in time, in which case it is tunable ONU.
The multi-wavelength TWMD-PON systems are standardized within the framework of the ITU-T G.989 series of Recommendations. The G.989 systems are supposed to support the following combinations of downstream and upstream line rates per each bi-directional wavelength channel:
10 G downstream and 10 G upstream
10 G downstream and 2.5 G upstream
2.5 G downstream and 2.5 G upstream
The TWDM-PON channel specifications for the line rate combinations involving the downstream line rate of 10 G are supposed to be derived based on G.987 XG-PON specification.
An ODN is characterized by the maximum fiber distance, that is, the overall length of fiber between the OLT and the remotest of the ONUs, and the maximum split ratio, that is, the smallest fraction of the optical power transmitted by the OLT (assuming no attenuation) that reaches an ONU due to the branching devices encountered in the ODN. The overall attenuation of an ODN, which depends of both the maximum fiber distance and the maximum split ratio, determines the loss budget of the ODN. For a passive optical system to operate correctly, the ODN loss budget should be balanced with the OLT and ONU transceiver optical power budget, which can be determined as a difference between the worst-case mean optical launch power of the transmitter and the worst-case receiver sensitivity, a parameter characterizing the minimum value of the received optical power at that the receiver is able operate. The receiver's ability to operate is usually quantified in terms of bit-error rate (BER) of received digital signal: generally, the weaker the received optical signal in its critical region, the higher the BER of the received digital signal. Therefore, the receiver sensitivity is not to be measured in the absolute terms, but rather with respect to a specified reference BER level. Normally, the reference BER level is set at between 10−10 and 10−12.
Forward Error Correction (FEC) is a well-known technique to improve the reliability of the data communication over an unreliable medium. It has been widely employed in data transmission and storage systems. In essence, to implement FEC, the source of data (that is, a transmitter in digital data communication, or a writer in digital data storage) adds redundancy to the data that is transmitted or stored. When the data is recovered by a receiver on a communication link, or by a reader of the storage device, the redundancy allows the decoder to detect and restore some of the data that may have been corrupted in the course of transmission or storage. The number of errors subject to successful restoration can be quantified and depend on the amount of redundancy added by the data source.
FEC in passive optical networks improves the BER of the received digital signal and, therefore, allows the use of less sensitive (and hence less expensive) receivers to balance the loss budget of a given ODN. Alternatively, FEC can be viewed as a tool to improve the optical power budget of the OLT and ONU transceiver pair. The use of FEC, however, comes at a cost of increased transmission overhead in the form of the digital bandwidth that is required to transmit the redundant information over the optical communication link.
In G.987 XG-PON1 systems operating at the line rates of 10 G downstream and 2.5 G upstream, the ONU receiver sensitivity is specified at the high reference level of BER=10−3, implying the use of FEC to reduce the effective BER to the required level of 10−12 after FEC is applied. Such BER reduction is possible if a high-redundancy FEC code is used. For such a high-redundancy FEC code, the ITU-T Recommendation G.987.3 specifies the Reed-Solomon code RS(248,216) which is a shortened form of the popular RS(255,223) code. The use of the shortened (or truncated) form of the code allows alignment of the size of the code word on the width of the system data path and to simplify the design of the system. An RS(248,216) code belongs to the family of systematic linear cyclic block codes. For each 216 symbols (bytes) of the useful data, it adds 32 bytes of redundant information (parity bytes).
Since in the downstream direction the FEC applies to all ONUs on the system, to account for the worst case ODN loss, ITU-T G.987 Recommendation series specifies the downstream FEC as always on. According to G.987.3, clause 10.3, “FEC support is mandatory for both OLT and ONU in the upstream as well as downstream directions. In the downstream direction, FEC is always on; in the upstream direction, the use of FEC is under dynamic control by the OLT.”
Since in the downstream direction the FEC applies to all ONUs on the system, to account for the worst case ODN loss, ITU-T G.987 Recommendation series specifies the downstream FEC as always on. According to G.987.3, clause 10.3, “FEC support is mandatory for both OLT and ONU in the upstream as well as downstream directions. In the downstream direction, FEC is always on; in the upstream direction, the use of FEC is under dynamic control by the OLT.”
An XG-PON1 system operates at the downstream line rate of 9.95328 Gb/s, transmitting a PHY frame of the size of 155520 bytes every 125 microseconds. A PHY frame consists of a 24 byte Physical Synchronization Block (PSBd) and sequence of 627 RS(248,216) code words, each code word being 248 bytes long. Therefore, the effective capacity of the XG-PON1 downstream link after FEC is 8.667648 Gb/s, or approximately 87.1% of the line rate. At the time when the XG-PON systems were standardized (2010), this effective capacity was considered sufficient for the envisioned applications. However, the newly emerging applications of XG-PON1 and, especially, of NG-PON2 TWDM-PON systems based on XG-PON1 may require low split ratio, but highest possible capacity. With low required split ratio, such applications do not operate under the worst case ODN losses, and, therefore, improving the power budget is no longer a priority. In such situations, incurring the FEC overhead may become an unnecessary burden. However, the always-on restriction and the lack of the downstream FEC ON-OFF control in XG-PON1 make that overhead unavoidable.
A known problem with dynamic FEC control is that turning FEC on and off involves processes that execute with different speed: multiplexing the FEC parity bytes onto the outgoing data stream or removing the parity bytes from the outgoing data stream can be achieved in effect instantaneously (a single PHY frame timescale). However, adjusting the rate of the datapath may require flow-control operations with extended feedback loops, and is relatively slow. For example, in the case of XG-PON1, the XGTC framing datapath (which is the next higher sublayer of the XG-PON protocol stack) handles data at the rate of 135432 byte per frame. Should it be possible to support FEC ON-OFF control, and disable FEC starting at a given PHY frame, the XGTC data path would have to transition to handling data at the rate of 155496 bytes per frame.
Such process speed discrepancy has led to the recognition that FEC ON-OFF adjustment may not be a lossless operation. Thus ITU-T Recommendation G.984.3 “G-PON TC layer specification”, which supports downstream FEC ON-OFF control and uses a single bit FEC indication emphasizes (G.984.3(2008), clause 13.2.3.1): “Note that the activation and deactivation of FEC is not meant to be an ‘in-service’ operation. The behavior during switch-over is undefined, and likely to cause a momentary loss of data.”